1. Field of the Invention
This invention generally relates to a blow molding method and a plastic product resulting therefrom, and, in particular, to a hollow plastic product partially different in composition and apparatus for manufacturing such a hollow plastic product. More specifically, the present invention relates to a method and apparatus for manufacturing a hollow plastic product which includes at least one section composed of a thermoplastic resin material which is different from a thermoplastic resin material forming the rest of the product, and a resulting hollow plastic product. More particularly, the present invention relates to a blow molding method and apparatus for manufacturing a hollow product which is different in composition at least partially and a resulting hollow plastic product.
2. Description of the Prior Art
It is well known to manufacture a hollow product of a desired shape by blow molding a parison extruded from a nozzle. However, in accordance with the prior art, use was made of a parison comprised of a single plastic material. Here, the single plastic material includes a uniform mixture of plastic materials, and, thus, the term "single plastic material" should be interpreted to include not only a plastic material composed of a single thermoplastic resin material, but also a plastic material which is, in fact, a mixture of two or more resin materials and which is uniform in composition. Thus, the term "single plastic material" indicates the fact that the composition is substantially uniform whether it is a mixture or not. According to the prior art technique, since use was made of a single plastic material, there was produced a plastic product which was uniform in characteristic. Thus, in order to vary the local characteristic of a resulting plastic product, the local thickness of the resulting product had to be made thinner or thicker; however, local thinning or thickening was not possible in some cases, so that the resulting product was rather limited.
It has been proposed to use a double-layer parison having two different plastic materials to manufacture a plastic product. In this case, however, each of the underlying and overlying layers is composed of a single plastic material, so that the overall characteristic of a resulting plastic product tends to be dominated by either one of the two overlying layers. Thus, applications of such a double-layer parison are also rather limited.